1. Field of the Invention
The present invention relates to inverter circuits that control emission of discharge tubes such as fluorescent lights, and particularly to inverter circuits that are able to realize controls in case of abnormal operations such as an open-circuited load abnormality, a short-circuited load abnormality, and the like with a reduced number of circuit components.
2. Description of the Related Art
As well-known, a discharge tube such as a fluorescent light emits light by driven with high frequency drive signals generated by an inverter circuit. This type of discharge tube is not only used for lighting purpose, but recently also frequently used for a light source of a backlight employed in a liquid crystal display apparatus. An inverter circuit and discharge tubes are configured such that a drive transformer is provided at an output side of an inverter included in the inverter circuit, and a fluorescent light is connected to a secondary coil terminal of the drive transformer by way of connectors.
However, it happens sometimes that the fluorescent light is not connected to the secondary coil terminal due to some defects in the connection terminals. Or, if output terminals of the secondary coils of the drive transformer are short-circuited with some reason, discharges may occur at the drive transformer by the high voltages, this may causes problems of smoking, firing, and the like. Other than the above-mentioned reason, if the fluorescent light breaks, or expires its life, output terminals of the secondary coils of the drive transformer connected to the fluorescent light become an open-circuited or a shot-circuited condition, so that this also cause above-mentioned problems of smoking, firing, and the like.
Accordingly, in the inverter circuit, it is necessary to avoid any heating up state in case of the abnormal operations such as the open-circuited load condition, short-circuited load condition. For the sake, conventionally, it is common to provide an abnormality operation detecting circuit that detects an open condition or short-circuited condition of the output terminals of the secondary coils in the inverter, and stops the operation of the inverter in case of the abnormal operation.
It is possible to detect current flowing through a discharge tube by connecting a detecting resistor at a low voltage side of the fluorescent light and by detecting the voltage across the resistor. Accordingly, the current flowing through the discharge tube is controlled to be constant by forming a feedback loop for the voltage across the resistor. Further, there are provided with an over-voltage protection circuit and an over-current protection circuit in the inverter circuit, which operate in case of abnormality in the discharge tubes.
For example, Japanese Patent Laid-open No. 2005-310587 discloses a circuit in which dividing capacitors are provided at secondary coils of a voltage-current conversion transformer, and an abnormal voltage detecting circuit for detecting abnormality of the discharge tubes is connected to the dividing capacitors. When a discharge tube is smashed, or broken, or when it becomes the open-circuited load condition due to non-contact of connectors, an output voltage increases, so that the abnormal voltage detecting circuit is able to detect the abnormality. On the contrary, when the connectors are short-circuited condition due to commingling of foreign particles into the connectors, the output voltage decreases, so that the abnormal voltage detecting circuit is also able to detect the abnormality.
However, it is necessary to provide the same number of abnormality detecting circuits as the number of the discharge tubes in the afore-mentioned conventional inverter circuit. Thereby, in case of inverter circuits for driving discharge tubes used in a larger LCD TV, the number of discharge tubes increases, so that the cost for the abnormality detecting circuits also resultantly increase. The conventional abnormality detecting circuit detects an increase of the detecting voltage when the discharge tube becomes an open-circuited condition, and detects the decrease of the detecting voltage when the discharge tube becomes a short-circuited condition. Accordingly, it is necessary to employ a circuit called a window comparator system. This window comparator circuit detects whether the detected voltage is between the upper limit and the lower limit or not. For the sake, this window comparator circuit requires two comparators. This also increases resultantly the number of components and costs.